This invention relates to xerographic developer units, and more particularly to a bearing seal for shafts which project through the housing of a xerographic developer unit.
Xerography and xerographic processes are used in various common reproduction devices. For instance, many presently marketed photocopiers and facsimile machines use xerographic processes in reproducing printed images. These xerographic devices generally include what is known as a developer unit.
The developer unit stores "developer", which is generally a particulate material made up of carrier particles and toner particles. The carrier is most commonly made from magnetite, which may be rough, spherical particles coated with a resin material. The toner is typically a polymer resin which includes a color agent and has a relatively small particle size compared to the carrier particles. The developer unit manipulates the developer so as to transfer toner onto a photoreceptor drum used in printing the reproduced image. The carrier particles actually carry the toner particles for proper application on the photoreceptor drum. However, when the toner is deposited on the photoreceptor drum, the carrier is retained in the developer unit.
Common developer units include a developer sump which is defined by a housing. Because the toner is used by the xerographic process, toner must be constantly replenished into the developer sump. A toner hopper will often be positioned adjacent the developer housing. Rotating paddles on a shaft are used to draw the toner out of the toner dispensing mechanism or toner hopper into the developer sump. To have the developer work properly, it is often desired that the toner and carrier particles be constantly stirred in the developer sump. This stirring is typically performed by an auger assembly on a second shaft which extends through the developer housing. A third shaft may also extend through the developer housing to support a magnetic roller which applies the developer to the photoreceptor drum.
It is generally desired to drive the paddle assembly, auger assembly and magnetic roller shafts with a motor located outside the developer housing. The ends of the shafts typically extend through the walls of the developer housing and end in a series of gears for connection to the motor.
For a xerographic device to work properly for an extended period of time, the developer unit housing must effectively seal the developer within the developer unit. Leakage of developer from the developer unit will adversely affect the performance of parts outside the developer housing and can result in failure of the xerographic device. For instance, the carrier particles are extremely abrasive. Leakage of carrier can cause wear of critical parts--including the developer unit drive train and parts within the paper feed section--which ultimately can lead to catastrophic failure of the xerographic device. Leakage of toner tends to create a mess within the xerographic device, and can accumulate on paper or other media being fed through the xerographic device as well as on the remaining elements within the xerographic device.
Accordingly, it is desired to have the shafts for the paddle assembly, auger assembly and magnetic roller extend through the developer housing but have a bearing and seal system for these shafts which will completely retain the developer within the developer unit housing. Typically, end journals of developer unit shafts are supported by stationary bushings with a bearing surface which supports the rotating shaft. Except for the portion of each end journal that fits into a bushing and the portion that protrudes outside the developer unit housing, the assemblies and seals are completely submerged in developer during unit operation. Leakage of developer into the contact area between the bushings and the shaft will cause wear of the shaft and/or bushing bearing surface, which will in turn accelerate leakage of developer from the developer unit. Consequently, it is crucial that the sealing system maintain constant contact to prevent developer from immediately falling into the critical interface between the bearings and end journals. Adequate sealing is most important on the geared end of the shafts, where the additional rotary force and stress the drive train places on the shafts causes seals to fail earlier and more frequently.
Various bushing and seal systems have been used to retain developer within the developer housing. For instance, in some photocopiers, a V-ring type seal is mounted on the shaft such that it rotates with the end journal of the shaft and bears against a radially extending inner face of the bushing. Other seal assemblies have existed which use a single stationary seal bearing against a rotating shaft. However, these previous systems have failed to perform effectively, and generally need to be replaced at intervals of 100,000 to 500,000 copy counts. If this suggested maintenance is not done, the seal/bearing assembly may fail, leading to leakage of developer and the aforementioned problems. Occasionally, failure of the seal may occur prior to the suggested maintenance limits, again causing serious problems which would be avoided by a better sealing system.
In response to developer leakage problems, polytetrafluoroethylene (PTFE) coatings have been applied to bushings and seals. These coatings are intended to allow the non-lubricated V-rings to rotate more smoothly against the bushing, thereby improving their seal and reducing wear. Various spacing systems have been utilized to more accurately position the seals against the bushings. Precise machining of the end journals of the shaft has been used to lessen any gap between the end journals and the bushings. While these lubrication coatings, spacers and machining have been helpful in extending seal life, leakage of developer remains a problem, and has occurred at copy counts as low as 100,000 copies. Additionally, some of the spacing systems require the service technician/engineer to determine exactly how many spacers to use, introducing human error as affecting seal effectiveness.